In recent years, a dramatic increase in the density and miniaturization of magnetic recording medium and magnetic memory device have progressed as a result of the development of a microscopic processing technology and the recording density has approximately arrived at a theoretical limit.
The direction of local magnetic moment M of a magnetic body corresponds to digital data of either “0” or “1” in such a magnetic recording medium or magnetic memory device.
A magnetic random access memory device (MRAM), which is an example of a magnetic memory device, is a memory device utilizing a change in the resistance value depending on the direction of the spin of electrons in a magnetic body as a result of a current flow in the magnetic structure, wherein GMR (giant magnetoresistance) elements or TMR (tunneling magnetoresistance) elements have been examined concerning the magnetic structure for the formation of memory cells [see for example, Japanese Unexamined Patent Publication 2003-031776 (Patent document 1) or Japanese Unexamined Patent Publication 2002-299584 (Patent document 2)].
Here, a great resistance change has been required in such an MRAM and therefore, the TMR element structure is primarily used in research and development.
When such a magnetic memory device or magnetic recording medium is formed by integrating magnetic units with a high density, the magnetic units are aligned in proximity to each other in the configuration of the magnetic memory device or magnetic recording medium, wherein the magnetostatic energy becomes the minimum when the opposite poles are alternately aligned in the case where the poles of magnetic bodies, that is to say N poles and S poles, are placed in proximity to each other.
The magnetic pole alignments other than the above gradually transit to the minimum energy condition due to thermal disturbance or as a result of a tunnel phenomenon and thereby, the recorded data disappears.
This disappearance of the recorded data in course of time is a critical defect in the magnetic recording medium or magnetic memory device and therefore, it is necessary to reduce as much as possible the magnetic interaction between magnetic units that hold data in order to prevent the above described disappearance of the recorded data due to the magnetic interaction.
As one effective method for the above, usage of nanoscale magnetic bodies in ring form, that is to say, usage of nanoring units as the recording units has been proposed [see for example, Japanese Unexamined Patent Publication 2001-084758 (Patent document 3) or Journal of Applied Physics, Vol. 87, No. 9, pp. 6668–6673, May 1st, 2000 (Non-patent document 1)].
See FIG. 10.
FIG. 10 is a diagram showing a conceptual configuration of a nanoring unit, wherein the nanoring unit is a ring with a diameter of approximately 100 nm fabricated from a ferromagnetic body such as permalloy (NiFe alloy) having a small magnetic anisotropy where a magnetic vortex structure (magnetic flux closure domain) is formed so as to contain magnetic flux indicated by arrows inside.
In such a magnetic vortex structure, the clockwise direction and the counterclockwise direction of magnetic flux have equal energy and therefore, magnetic memory cells are formed in a manner where the direction of rotation corresponds to digital data of either “0” or “1.”
This magnetic vortex structure does not have flux leakage where magnetic interaction between nanoring units is extremely small and accordingly, the data written in a nanoring unit is stably retained even in the case where nanoring units are aligned with a high density allowing the achievement of a recording density of approximately 400 Gbit/in2 (≈62 Gbit/cm2) which is a recording density ten times, or more, higher than the present recording density.
As described above, a ferromagnetic nanoring unit has excellent characteristics as a magnetic recording medium or magnetic memory device while the clockwise direction and counterclockwise direction of flux have equal energy and therefore, it is necessary to control the direction of rotation of magnetic flux for practical usage.
See FIGS. 11(a) to 11(c).
This is because whether the counterclockwise direction shown in FIG. 11(b) or the clockwise direction shown in FIG. 11(c) is gained cannot be controlled by adjusting energy during the process of conversion of the opposed domain structure formed by applying the external magnetic field shown in FIG. 11(a) into the magnetic vortex structure when the magnetic field is reduced to 0.
Thus, in the above described Patent document 3, a non-magnetic conductor is provided in order to penetrate through the center of a ferromagnetic nanoring unit and the direction of rotation is regulated by the direction of current that flows through this non-magnetic conductor.
In addition, an antiferromagnetic pattern is locally provided at a position that shifts from the rotational symmetry on the surface of the ferromagnetic nanoring unit so that the direction of magnetization of the pinned layer is fixed due to the direction of magnetization provided to this antiferromagnetic pattern.
In addition, another method has been proposed wherein a constriction, or the like, is provided in a nanoring so that the direction of rotation is controlled by pinning magnetic domain walls [see for example, Applied Physics Letters, Vol. 78, No. 21, pp. 3268–3270, May 21st, 2001 (Non-patent document 2)].
In the above described Patent document 1, however, it is necessary to make the insulation between a feed-through conductor and a nanoring complete, and to do so, it is necessary for an insulating film without a pinhole to be formed so as to have a sufficient thickness that can prevent a tunnel phenomenon and in addition a problem arises where an antiferromagnetic pattern is required.
In addition, a problem arises in the above described Non-patent document 2 where a unit is thermally agitated as a result of the utilization of effects of pinning magnetic domain walls and therefore, a stable operation cannot be expected at room temperature.
Accordingly, an object of the present invention is to control the direction of rotation of the magnetic flux freely and with high reproducibility in a simple structure where a thermal process such as pinning is not used.